In exercising muscles, alpha-adrenergic vasoconstriction is sensitive to inhibition by local metabolic products of skeletal muscle contraction. Termed functional sympatholysis, such metabolic modulation negates an otherwise deleterious effect of sympathetic activation on muscle perfusion. In the last grant cycle, we provided multiple lines of evidence in rodents and humans that nitric oxide (NO) is a key mediator of functional sympatholysis. Remarkably, the primary source of the NO involved in sympatholysis appears to be the neuronal isoform of NOS (nNOS) that is highly enriched in skeletal muscle. We now hypothesize that the bioavailability of skeletal muscle-derived NO at the vascular alpha-adrenergic receptors is determined by two major intracellular mechanisms: (1) localization of nNOS to the sarcolemma by the dystrophin complex which couples Ca2+-dependent activation of nNOS to the intensity of muscle contraction and facilitates NO diffusion across the sarcolemma; and (2) the extent to which NO is inactivated by reactive oxygen species, such as superoxide (O2) produced in skeletal muscle. Because angiotensin II (Ang II) is a potent stimulus for O2-superoxide production, we hypothesize that increased generation of O2-superoxide is a common mechanism impairing sympatholysis in pathophysiologic states characterized by activation of the renin-angiotensin system. We further hypothesize that in such states the resultant unopposed alpha-adrenergic vasoconstriction in exercising muscle produces functional muscle ischemia, triggering exaggerated reflex increases in sympathetic nerve activity and blood pressure. To test these hypotheses, we will use complementary in vivo rodent and human models. To manipulate the quantity and cellular localization of nNOS in skeletal muscle, we will use genetic mouse models and adenoviral gene transfer. To increase O2-superoxide production by Ang II, we will use several models including prolonged Ang II infusion and nitrate tolerance in both rats and humans. The distinctive features of this proposal include: (1) the integration of animal and human studies; (2) the use of adenoviral gene therapy of rodent skeletal muscle to test mechanistic hypotheses about local vascular regulation; (3) the novel hypothesis linking vascular regulation by NO with subcellular localization of nNOS in skeletal muscle; (4) the novel hypothesis implicating a pivotal role for skeletal muscle-derived O2-superoxide in producing functional muscle ischemia in common pathophysiologic states characterized by activation of the renin-angiotensin system, and (5) the exciting potential clinical implications of this hypothesis-driven research.